1. Field of the Invention
The present invention relates to the trimerization of organic isocyanates using an improved catalyst.
2. Description of the Prior Art
The trimerization of organic isocyanates is a known reaction. As catalysts for the trimerization of organic isocyanates there are described in the literature a great number of chemically very diverse compounds. Thus, there can be used as trimerization catalysts metal compounds from the salt and base groups and homopolar metal compounds, like naphthenates, Na-benzoate in dimethylformamide (DMF), alkaline-earth acetates, formates and carbonates, metal alkoxides, AlCl.sub.3 and Fe-acetylacetonate. These are only partially separable, if at all, from the isocyanurate reaction products and only with great difficulty. The catalysts more likely remain in the reaction product.
This is naturally a great disadvantage, especially when one has to deal with an isocyanurate produced by the trimerization of a diisocyanate. In heating such products, the free NCO-groups of these isocyanurates continue to react in the presence of the trimerization catalyst. Higher molecular weight polyisocyanates containing isocyanurate rings are formed. With advancing reaction there is even cross-linking. Thus it is clear that catalyst-containing isocyanurates, which also contain NCO-groups bound by alkylene groups to the triazine ring, can hardly be used for the desired further reactions with alkohols or amines.
The aim in any process leading to isocyanurates (from polyisocyanates) must therefore be the complete elimination or neutralization of the catalyst after the trimerization. This can be achieved by using readily volatile basic compounds, like tertiary amines or phosphines, as trimerization catalysts. The disadvantage in using these compounds is, of course, that they (tertiary phosphines) catalyze not only the trimerization but also, in part, the dimerization of the isocyanates. Further, it must be noted that tertiary amines while catalyzing very well the trimerization of aromatic isocyanates, show no catalytic effect for aliphatic or cycloaliphatic isocyanates. Thus, for example, the trimerizations of hexamethylenediisocyanate (HDI), 2,2,4- or 2,4,4-trimethylhexamethylene diisocyanate-1,6 (TMDI); 3-isocyanatomethyl-3,5,5-trimethyl-cyclohexylisocyanate, also called isophorone diisocyanate (IPDI), or 3(or 4), 8(or 9)-diisocyanato-methylnorbornane (NDI) suceed neither with triethylamine nor with dimethylbenzylamine nor with 1,4-diazabicyclo[2.2.2]octane. These aliphatic or cycloaliphatic isocyanates however can be readily trimerized with Na-benzoate in DMF (Chemical Abstr. 60, 8332 (1963)) or with Na-phenolates in n-butylacetate (GB-PS 1,386,399). The great disadvantage of this just-mentioned method is the catalyst elimination from the reaction product. Na-benzoate as well as Na-phenolate must be filtered out from the reaction product after trimerization of the diisocyanate. This filtration leads to difficulties inasmuch as one has to deal with very finely crystalline (Na-benzoate particle sizes &lt;2.mu.) relatively slimy (Na-phenolate) precipitates.
A need therefore continues to exist for a method for producing 2, 4, 6-triketohexahydrotriazines by catalytic trimerization with a catalyst that does not give rise to the aforementioned problems.